Correlation between microstructure evolution and fluorescence properties of functionalized multi-walled carbon nanotubes

The fluorescence properties of multi-walled carbon nanotubes (MWCNTs) depend strongly on the microstructure, while their specific surface area increases dramatically at oxidation temperatures above 75 ℃. In this work, the microstructure evolution of MWCNTs was carefully studied by oxidation and amination over a narrow temperature range to clarify its effect on the fluorescence properties. Oxidation was performed by refluxing the MWCNTs in mixtures of sulfuric acid and nitric acid at the temperature from 75 to 95 ℃. After the oxidation treatment, the MWCNTs were further aminated with ethylenediamine. The microstructure evolution at each stage was determined by the results from transmission electron microscopy, Raman spectroscopy and Fourier transform infrared spectroscopy. The results indicate that the microstructure undergoes significantly changes over this temperature range. With the increase of temperature, the oxidized nanotubes are gradually broken and introduced defect states, but the amidation degree and fluorescence emission show a trend of first increasing and then decreasing. The MWCNTs oxidized at 85 ℃ possess the tubular structure with the length of 100–200 nm and a large number of defect states, and exhibit the optimal fluorescence efficiency after amination. What’s more, the possible mechanisms involved in the effect of microstructure evolution on fluorescence were also discussed.